Protein-calorie malnutrition is the most common nutritional deficiency disease in the world. Children so afflicted are often retarded in mental development. It is unclear whether the retardation is a direct influence of malnutrition or the generally impoverished environment within which these children are raised or a combination of these factors. The role that calories and protein play in the etiology of the disease and whether deficiencies of these have similar effects upon development is unknown. The age at which the deficiency occurs relative to the stage of brain development may be important in determining the outcome of the deficiency. This study will utilize baby cebus and squirrel monkeys to define the characteristics of protein and calorie deficiencies, to sort out the roles played by nutrition and psychological environment in mental development, and the interactions between these factors. Groups of animals will be deprived of either protein or calories (levels which permit survival but severely limit growth) and they will be raised in either psychologically impoverished (relative isolation) or enriched environments (group rearing). Studies will be begun at either an early age (2 weeks) or at 8 weeks of age when brain growth approaches adult size. Batteries of tests are being developed which will be utilized at appropriate intervals to evaluate development. A variety of biochemical tests will also be applied to compare the effects of the deficiencies. The response to protein deficiency may be dependent upon the kind of protein in the diet. The kind as well as the level of protein may also determine the response to caloric deprivation. Studies are planned to investigate essential amino acid requirements and the effects of caloric deprivation when both the quality and amount of dietary protein are varied. These studies will determine the kinds of dietary environments worthy of inclusion in the nutritional-psychological investigations in the future.